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The Eater of Worlds must be spawned in a Corrupted environment. There are certain requirements of a number of Ebonstone blocks for an area to be considered Corrupted. I'm attempting to find the exact numbers, but I believe the 200+ is correct. When you're in a Corrupted environment, the music and background will change. By watching/listening for that, you can know whether you can spawn the Eater of Worlds.